The chemical fixation and transformation of carbon dioxide (CO2) has attracted much attention in view of environmental problems, legal and social issues in the past decades. Carbon dioxide is an attractive C1 building block in organic synthesis as it is an abundant, renewable carbon source and an environmentally friendly chemical reagent. The utilization, as opposed to storage, of CO2 is indeed more attractive especially if the conversion process to useful bulk products is an economical one. Unfortunately, carbon dioxide is a highly oxidized and thermodynamically stable compound, and is consequently not very reactive. Significant efforts have been devoted towards exploring technologies for CO2 transformation, however harsh and severe reaction conditions are one of the major limitation for their practical applications. Therefore, the development of efficient catalyst systems for CO2 utilization under mild conditions is highly desired, especially for real world applications.
Carboxylic acids are one of the most important types of compounds in medicinal chemistry and also in fine chemicals synthesis. Although there are many well-established protocols for the preparation of carboxylic acids, the direct carboxylation of carbon nucleophiles using CO2 as the electrophile is the most attractive and straightforward method. While the formation of a stable C—C bond is desired for CO2 fixation, its reality remains hitherto unaccomplished, and remains the most challenging aspect thus far. Typically, this type of reactions is facilitated by the insertion of CO2 into a metal-carbon bond, as shown in Scheme 1.

Recently, this protocol was expanded to less reactive organometallic reagents where catalytic insertion CO2 into less polarized metal-carbon bonds (C—Sn, C—Zn, C—B) with high chemoselectivity and under mild reaction conditions was achieved. Generally, such reactions take place under mild reaction conditions. However widespread use of these methods is limited by the synthesis of related organometallic reagents as precursors and the restricted substrate scope. In the past decades, several interesting systems have been reported for metal mediated reductive carboxylation of alkenes and alkylenes with CO2 to form carboxylic acids or esters. However, most of those systems need either stoichiometric amount of transition metals as reactants or excess amount of organometallic reagents for transmetallation processes. The development of catalytic systems for synthesis of carboxylic acid product by direct CO2 carboxylation without using stoichiometric organometallic reagents is highly desired, especially for commercial applications. An alternative possibility to achieve the catalytic synthesis carboxylic acid from CO2 is by direct C—H bond activation and carboxylation, but unfortunately, this is still an undeveloped field.
Previously known methods for the preparation of alkynyl carboxylic acids include CO2 insertion into metal-carbon bond of organometallic reagents, hydrolysis of bromide and related derivatives and the oxidation of preoxidized substrates, such as alcohols or aldehydes. Despite the efficiency of these conventional procedures, they are restricted by the severe reaction conditions and the organometallic reagent that dramatically limited the synthesis and application of a wide scope of functionalized propiolic acids.